1. Field of the Invention
The present invention concerns the field of atomic clocks and especially, but not exclusively, cesium beam frequency standards.
2. Description of the Prior Art
Many atomic frequency standards have been proposed. The following documents describe, for example, standards of this type: FR-A-1 287 180, FR-A-1 446 675, FR-A-1 594 565, FR-A-2 163 610, FR-A-2 206 539, FR-A-2 224 901, FR-A-2 316 836 and FR-A-2 327 671.
Essentially, atomic frequency standards generally comprise:
a quartz oscillator, PA1 a tube containing a material (most usually an alkaline metal such as cesium or rubidium), the atoms of which have an hyperfine spectral transition; PA1 control means capable of generating, from the quartz oscillator, a control signal of a frequency that corresponds to the hyperfine spectral transition, and of applying this control signal to the tube to cause interaction between this signal and the atoms of the material contained in the tube; and PA1 feedback means that are sensitive to the response of the tube and adapted to modifying the frequency of the quartz oscillator so as to substantially center the frequency of the control signal on the frequency of the hyperfine spectral transition. PA1 these conventional standards are sensitive to the environment, notably to temperature; PA1 the linearity of the sinusoidal frequency modulator at 137 Hz is difficult to control; PA1 the settings to be made are many and complicated; PA1 the overall cost of the devices is high.
In connection with cesium tubes, it is common to use the hyperfine resonance transition that occurs between the states F=4, nF=0 on the one hand, and F=3 and mF=0 on the other hand. The definition of these states is explained in the above-mentioned documents, especially in the documents: FR-A-1 594 565 and FR-A-2 316 836.
To cause a transition from one of these states to the other, the atom should either give or absorb a quantity of energy E equal to the difference in energy between the above-mentioned states. The frequency f of the control signal required to cause a change in state is given by the equation f=E/h where h represents Planck's constant.
The value of f for the transition of cesium of F=4, mF=0 to F=3, mF=0 is 9 192 631 771.59 Hz. This value of f shall hereinafter be called Fr.
The tubes containing the material, the atoms of which have an hyperfine spectral transition, are known per se and shall therefore not be described in detail hereinafter. It will be noted, however, that cesium tubes generally comprise: a furnace generating a beam of cesium atoms through a collimator, a microwave cavity at which the control signal is coupled to the stream of cesium atoms, magnetic state selecting means placed on either side of the cavity and a detector unit comprising an ionizer, an accelerator, a mass spectrometer and an electron multiplier. The electron multiplier delivers an output current which is proportionate to the number of atoms arriving at the ionizer and, therefore, proportionate to the number of atoms which have been brought to the selected state in the microwave cavity.
The resonance signal obtained at the output of the tube, as a function of the frequency of the control signal applied, is shown schematically in the appended FIGS. 2, 3 and 4.
The signal shown in FIG. 3 is generally called the Ramsey response. It represents the total transfer function of the tube.
The signal has a maximum amplitude when the frequency of the control signal applied is equal to the frequency of the hyperfine spectral transition Fr (namely, 9 192 631 771.59 Hz in the case of cesium). The peak centered on this frequency Fr is called the Ramsey peak. This peak is symmetrically framed by dampened stray oscillations of smaller amplitude (see FIG. 3). The base line is linked to these dampened stray oscillations by variably sloping, symmetrical base sides, generally called the "Rabi" of Ramsey.
The frequency band .DELTA.Fr (see FIG. 3) covered by the central Ramsey peak depends directly on the length of the stream of atoms and, hence, on the length of the tube. The longer the tube, the smaller is the width of the Ramsey peak.
Certain atomic frequency standards used in laboratories are 5 m long, thus making it possible to reduce the width of the Ramsey peak .DELTA.Fr to some 30 Hz. Industrially used atomic frequency standards, especially in the context of mobile applications, are far shorter in length, and the width of the Ramsey peak .DELTA.Fr is generally of the order of 1 KHz.
The Ramsey response further includes auxiliary resonance phenomena, symmetrically on either side of the central Ramsey peak. These auxiliary resonance phenomena appear at regular frequency intervals. The amplitude of these auxiliary resonance phenomena is all the weaker as they are distant from the Ramsey peak. Each auxiliary resonance placed on either side of the Ramsey peak, which are closest to this peak, has a peak of greater amplitude (which remains smaller than the Ramsey peak) called the Zeeman peak, dampened stray oscillations symmetrically framing the Zeeman peak and variably sloping base sides, generally called the Rabi of Zeeman, which link the dampened stray oscillations to the base line.
To produce a signal which can be used to command the feedback controlled oscillator, the frequency of the control signal applied to the microwave cavity is modulated around the central atomic resonance frequency Fr corresponding to the Ramsey peak.
In practice, in atomic frequency standards being marketed at present, the control signal is modulated by a low sinusoidal frequency at 137 Hz on a depth .+-.250 Hz, namely, the width of the Ramsey peak at mid-amplitude.
The atomic frequency standards proposed up till now have already been very useful. However, they have different drawbacks.
The main drawback of conventional standards would appear to be the fact that the checks on the acquisition of the central peak of the Ramsey response are unreliable.
Other secondary drawbacks in conventional standards seem to be due to the fact that: